ld7576a Leadtrend Technology, ld7576a Datasheet - Page 10

• Manufacturer Part Number: ld7576a
• Description: Green-mode Pwm Controller With High-voltage Start-up Circuit And Adjustable Olp Delay Time
• Manufacturer: Leadtrend Technology

Current Sensing, Leading-Edge Blanking
and the Negative Spike on CS Pin
The typical current mode PWM controller feeds back both
current signal and voltage signal to close the control loop
and achieve regulation. The LD7576A detects the primary
MOSFET current from the CS pin, which is not only for the
peak current mode control but also for the pulse-by-pulse
current limit. The maximum voltage threshold of the
current sensing pin is set at 0.85V. Thus the MOSFET
peak current can be calculated as:
A 230nS leading-edge blanking (LEB) time is provided in
the input of CS pin to prevent false-triggering from a
current spike. In low power applications, if the total pulse
width of the turn-on spikes is less than 230nS and the
negative spike on the CS pin does not exceed -0.3V, the
R-C filter (as shown in figure15) can be eliminated.
However, the total pulse width of the turn-on spike is
related to output power, circuit design and PCB layout. It
is strongly recommended to add a small R-C filter (as
Leadtrend Technology Corporation
LD7576A-DS-00b November, 2009
UVLO(on)
UVLO(off)
HV Current
Vcc
Vcc current
Startup Current
(<100uA)
1mA
I
PEAK
(
MAX
Fig. 14
)
Operating Current
(Supply from Auxiliary Winding)
=
. 0
R
~ 0mA (off)
85
S
V
www.leadtrend.com.tw
t
t
10
shown in figure 16) for higher power applications to avoid
the CS pin from being damaged by the negative turn-on
spike.
Output Stage and Maximum Duty-Cycle
An output stage of a CMOS buffer, with typical 500mA
driving capability, is incorporated to drive a power
MOSFET directly. The maximum duty-cycle of LD7576A
is 75% to avoid the transformer saturation.
Voltage Feedback Loop
The voltage feedback signal is provided from the TL431 on
the secondary side through the photo-coupler to the
COMP pin of LD7576A. The input stage of LD7576A, like
the UC384X, is with 2 diodes voltage offset to feed the
voltage divider with 1/3 ratio, that is,
A pull-high resistor is embedded internally. Generally, an
external capacitor in parallel to photo-coupler is required in
application.
LD7576A
V
VCC
+
(
PWM
GND
OUT
COMPARATOR
CS
Fig. 15
)
Can be removed if the negative
spike is not over spec. (-0.3V).
=
3
LD7576A
1
×
(
V
COMP
230ns
blanking
time
−
2
V
F
)